The present invention relates in general to control of an actuator and, more particularly, to a method and apparatus accurately controlling the velocity of the actuator member by monitoring the back electromotive force (xe2x80x9cEMFxe2x80x9d) of a actuator coil, and driving the coil with a voltage.
Conventional actuators sometimes referred to as xe2x80x9cmotorsxe2x80x9d, have a movably supported member and a coil. When a current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such an arrangement is found in a hard disk drive, where the movable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement of the head relative to the disk. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. In such a situation, it is desirable to move the member to a parking location, where it is held against potentially damaging movement which could occur if the member were not so parked. The movement of the member to the parking location is commonly referred to as a retract of the member.
When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-EMF voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators:
VM=IM*RM+Kexcfx89
VM=voltage across actuator (motor),
IM=current through actuator
RM=internal resistance of actuator
Ke=torque constant of actuator, and
xcfx89=velocity of actuator.
The term, Ke xcfx89, represents the back-EMF of the actuator coil.
Apparatus have been provided that control such actuators by providing a drive current to the coil of the actuator in response to the provision of a target speed voltage signal having a voltage corresponding to the target speed of the moveable member. For example, co-pending patent application U.S. Pat. No. 6,040,671, issued on Mar. 21, 2000, and entitled xe2x80x9cCONSTANT VELOCITY CONTROL FOR AN ACTUATOR USING SAMPLED BACK EMF CONTROL,xe2x80x9d discloses such an apparatus. However, such apparatus does not lend itself readily to providing such control in cases where the drive transistors for the actuator are power MOSFETs external to the integrated circuit (xe2x80x9cICxe2x80x9d) containing the control circuitry. In such cases, it is difficult and/or expensive to implement a current mode output. To do so would require current feedback. To process this feedback, additional circuitry would be required. This additional circuitry would add expense and would be difficult to operate at low voltages such as experienced with the power failure.
FIG. 1 is a diagrammatic view of a system including an actuator 10 under control of a control circuit 12. The particular system shown is that of a hard disk drive, in which the actuator 10 controls the movement of a member 20 on which a read/write head 34 is mounted. The control circuit 12 applies drive signals DRV+ on line 14 and DRVxe2x88x92 on line 16 in response to a move command voltage signal VC on line 18. The drive signals DRV+ and DRVxe2x88x92 cause motion in a member 20 of actuator 10 by setting up a force field in a coil 22 on the member 20. The force field thus set up in coil 22 interacts with the magnetic field of a permanent magnet 24 disposed nearby. Member 20 is constrained to move about a shaft 26, resulting in pivoting motion as shown by arrow 28. The member is constrained in its movement between a first stop 30 and a second stop 32. The result is that a magnetic head 34 is caused to move about a magnetic disk (not shown in this figure) in conjunction with the reading and writing of data from and to the magnetic disk in a hard drive system.
FIG. 2 is a high-level block diagram of a control unit and the actuator it controls, such as is used in the system shown in FIG. 1. A control circuit 90 receives a move command signal VC on line 92 and provides drive current DRV+ and DRVxe2x88x92 to an actuator. In FIG. 2, the actuator shown is an idealized model 65 of an actuator. It will be appreciated that the control circuit 90 would be unable to xe2x80x9cseexe2x80x9d a significant difference between the actuator model 65 and an actual actuator, were an actual actuator connected to control circuit 90.
The actuator model 65 includes an ideal current sensor 66, an inductance 68, a resistance 70 and an ideal voltage-controlled voltage source 72, all coupled in series between the two terminals 94, 96 of the actuator model 65. The output 67 of the ideal current sensor 66 is a signal representing the current flowing through the actuator. This signal 67 is coupled to an input of an amplifier 74, which has a gain Kt that represents a torque constant of the moveable member 20 (FIG. 1). The output of the amplifier 74 is coupled to the input of a junction 76, which adjusts the amplifier output using a signal representing a load torque. The output of junction 76 is coupled to the input of a circuit 78, which makes an adjustment representative of the inertia J, of the member 20.
The output 80 of the circuit 78 is a signal which represents an acceleration of the member 20. The signal 80 is integrated at 82, in order to obtain a signal 84 which represents the velocity of the member 20. The signal 84 is applied to the input of an amplifier 86 having a gain Ke that represents an electrical constant for the back-electromotive force (EMF) of the actuator. The output 88 of the amplifier 86 is a voltage Vbe which represents the back-EMF voltage of the actuator. This voltage is applied to an input of the ideal voltage-controlled voltage source 72, which reproduces this same voltage Vbe across its output terminals. Since the voltage source 72 is ideal, it produces the output voltage regardless of whether there is any current flowing through source 72.
Since the signal 84 represents the actual velocity of the member 20, and since the back-EMV voltage Vbe present at 88 and across source 72 is proportional to the magnitude of signal 84, it will be appreciated at the magnitude of the back-EMF voltage Vbe across source 72 is an accurate representation of the actual velocity of the member 20. However, when a current is flowing through the actuator model 65, the resistance 70 produces a voltage which is added to the voltage Vbe across the voltage source 72. Consequently, so long as current is flowing through the actuator model 65, it is not possible to accurately measure the voltage Vbe alone, in order to accurately determine the actual velocity of the movable member.
Therefore, the system of FIG. 2 independently measures the back-EMF voltage Vbe, and thus determines the actual velocity of the member 20. It does this by interrupting the current flow through the actuator coil 68 so that the voltage across the resistance 70 goes substantially to zero, after which the back-EMF voltage Vbe is measured across the two terminals 94, 96, of the actuator model 65. It is a characteristic of the actuator that the back-EMF voltage Vbe does not change rapidly after the current flow through the actuator model 65 is decreased to zero, once short term transient effects have died down.
The control circuit 90 includes the following components. A junction 98 receives the retract command voltage signal VC on line 92 that corresponds to a target velocity for the actuator member 20. The output of junction 98 is provided to a proportional compensation unit 100 that provides a proportional amplification to the input provided thereto. Thus, the output of unit 100 is some multiple of the input, i.e., unit 100 is substantially a linear amplifier. Of course, the proportional factor in unit 100 may be one, in which case the output would be the same as the input.
The output of terminal 98 is also provided to an integral compensation unit 102, which provides a mathematical integration operation on its input to derive its output. The output of unit 100 provided to one input of terminal 104, while the output of unit 102 is provided to another input of terminal 104. The outputs of units 100 and 102 are added in terminal 104, and the output, which is a voltage the level of which represents a commanded current level, ICMD, is provided on line 106 to a transconductance linear amplifier 108. The outputs of amplifier 108 are the differential drive currents DRV+ and DRVxe2x88x92 which are provided on lines 110 and 112, respectively. The DRV+ signal is synchronous with a DRIVE control signal. Lines 110 and 112 are provided to input terminals 94 and 96, respectively, of the actuator model 65. Lines 110 and 112 are also connected to the differential inputs of a voltage sense unit 116. The output of the voltage sense unit 116 is provided to a sampler unit 118. A timer 120 generates two timing signals, a FLOAT timing signal which is applied to transconductance amplifier 108 and a SAMPLE timing signal which is applied to sampler unit 118. The output of sampler unit 118, on line 122 is provided to a second input to terminal 98. The signal on line 122 is subtracted from the signal on line 92 in terminal 98.
The operation of the control circuit 90 of FIG. 2 may be better understood by reference to the signal timing diagram shown in FIG. 3. FIG. 3 shows the FLOAT timing signal, the SAMPLE timing signal, and the DRIVE signal, all mentioned above. These three signals are presented along a common horizontal time axis, and so their relative timings may be easily seen. As can be seen in FIG. 3, the FLOAT signal is a regularly recurring rectangular pulse. Looking now at one set of pulse signals, at timing 130 the FLOAT signal begins.
This causes amplifier 108 (FIG. 2) to turn off the drive signals, as can be seen by looking at the signal DRIVE in FIG. 3. After sufficient time for the transient effects in inductor 68 (FIG. 2) of the actuator to die down, at timing 132, a SAMPLE pulse begins. A SAMPLE pulse is provided for a sufficiently long period of time to enable the sampler unit 118 (FIG. 2) to sense the voltage at the output of amplifier 116. At time 134 the SAMPLE pulse ceases. After a small delay, at time 136, the FLOAT signal ends. A short time thereafter, at time 138, the drive signals resume. The sequence thus described repeats regularly.
Thus, in operation, the command voltage VC is provided on line 92 to terminal 98. There, it is combined with a voltage on line 122. The output of terminal 98 is provided to the proportional compensation unit 100 and integral compensation unit 102, the outputs of which are combined in terminal 104 to yield the current command signal ICMD. The current command signal ICMD is converted into actual drive currents by the transconductance amplifier 108, to yield the drive currents DRV+ and DRVxe2x88x92 which are applied to the terminals 94 and 96, respectively, on the actuator model 65. At the same time, the voltage across terminals 94 and 96 is sensed by voltage sense unit 116. The timer unit 120 applies the FLOAT signal to amplifier 108, thus interrupting the drive current, a short time after which the SAMPLE signal is provided to sampler unit 118, which samples and stores the voltage output from voltage sense unit 116, thus the back-EMF voltage, undisturbed by voltage effects produced by the application of the drive currents, is sensed and stored in the sampler unit 118 on a regularly occurring basis. This sampled and held voltage is provided on line 122 to the terminal 98 where it is subtracted from the command voltage VCMD to yield a feedback-corrected control voltage.
The feedback-corrected command voltage is then applied to the proportional compensation unit 100 and the integral compensation unit 102. As mentioned above, the proportional compensation unit 100 provides an output that is some multiple of its input. This multiple may be unity. The purpose of the proportional compensation unit 100 is to shape ICMD so as to enable the control circuit 90 to respond better to large errors in the actual velocity, as compared with the desired, commanded velocity, while ensuring stability in the control circuit 90. This is desired because, for example, in a retract operation, the situation in which the retract is initiated may be in the middle of a hard drive xe2x80x9chard seekxe2x80x9d operation. In a hard seek; the actuator coil is driven to the point of maximum velocity so as to rapidly move the head to a desired track on the hard drive. The voltage corresponding to this velocity might be, say, 7 Volts. By contrast, an exemplary voltage corresponding to a desired retract operation speed may be, say, one volt. The proportional compensation unit 100 allows the control circuit 90 to immediately respond to this wide disparity between actual speed and desired speed, without destabilizing the system. In selecting a suitable value for the proportional amplification factor, the practitioner should keep stability foremost, and set a bandwidth that is significantly less than the frequency of the SAMPLE signal pulses, while allowing relatively quick control of the actuator.
The integral compensation unit 102, as mentioned above, provides a mathematical integration operation on its input to derive its output. Thus, its response is slower than the proportional compensation unit 100, and is unsuitable for reliance to respond to large errors in velocity, such as described above. This is why the proportional compensation unit 100 is provided. However, the proportional compensation unit 100 is not optimal for response to large changes in the torque load that the actuator member may encounter. In such situations, the proportional compensation unit 100 is inadequate to maintain the desired relatively constant velocity. By contrast, the integral compensation unit 102 does respond well to even large and abrupt changes in torque load. When such a large torque load change is encountered, the integral compensation unit 102 gradually integrates the change in resultant velocity that the torque load change is inducing, and steadily increases the compensating current command to maintain the velocity constant. The result is adequate magnitude compensating current command, without destabilization of the control circuit 90. The control circuit 90 has another implementation presented on FIG. 4. The FIG. 5 illustrated a possible circuit diagram for the solution presented in FIG. 4.
The circuit of FIG. 5 employs two capacitors namely capacitor 294 and capacitor 316. These two capacitors 294 and 316 use a lot of area when formed on a semiconductor device. Additionally, each capacitor 294 and capacitor 316 requires a set of switches to switch the capacitors 294 and 316 in and out. This causes charge loss and as a consequence of the charge loss causes large errors.
Accordingly, it is desired to have an apparatus that controls actuators using a voltage mode output. Such a configuration would eliminate the requirement for current sensing and should, therefore, be simpler and less expensive to build.
The present invention reduces the number of capacitors needed in a control circuit to one and consequently reduces the amount of the area on an integrated circuit. Additionally, the present invention reduces the voltage requirements of the control circuit because it reduces the charge loss by use of a single capacitor. Using more than one capacitor increases the number of capacitors and correspondingly the charge loss. The single capacitor holds the delta or difference voltage and not the plus and minus voltage on separate capacitors.